Cost and size of engine components are of constant concern in vehicular engine applications. This is particularly true for small turbojet engines that are designed for use in missiles and other short-life/disposable applications.
It is known to use a pulse width modulated valve (PWM valve) on the high pressure side of a fuel pump to meter the fuel flow to a gas turbine engine by cycling the PWM valve between an on and off position. Fuel flow is determined by the time period that the valve is open during each cycle and by the cycle frequency. Typically, such systems utilize a regulator valve to control the inlet pressure to the PWM valve by bypassing fuel flow from the high pressure side of the fuel pump back to the fuel tank. Examples of such systems are shown in U.S. Pat. Nos. 3,568,495 to Fehler et al.; 3,936,551 to Linebrink et al.; and 4,015,326 to Hobo et al.
Two disadvantages associated with these systems are the size and cost of the PWM valve components which must be designed to withstand the output pressure of the fuel pump, which commonly is in the range of 100-200 psig to provide adequate fuel injection pressure to the combustor.
Another disadvantage associated with these systems is the wasted power input into the pressurized fuel flow that is bypassed by the regulator valve from the high pressure side of the fuel pump back to the fuel tank. The wasted power is particularly critical in missiles and other vehicles having a limited fuel capacity and a mission profile that may be determined by the time required to deplete the stored fuel.
Yet another disadvantage associated with these systems is the pulsating flow generated by the PWM valve as it cycles between its open and closed positions. Such pulsating flow can result in combustor flameout and/or deleteriously affect the combustor stability. Accordingly, depending on the engine and combustor parameters, these systems typically require some form of accumulator/damper in the high pressure fuel line connecting the PWM valve to the combustor to dampen the pulses in the fuel flow to the combustor. The accumulator/damper is an additional component that adds cost, complexity and weight to the system and introduces a potential failure point in the system.
Thus, it can be seen that there is a need for a small, low-cost, and efficient fuel control system for gas turbine engines and, in particular, for small turbojet engines.